Water-absorbent resin has been widely used for hygiene products such as disposable diaper and sanitary products, products for daily use such as sheets for pets, industrial materials such as water-absorbent pads for food, water-blocking materials for cables, and dew-catcher, water retention agents for greening, agriculture, and gardening, soil improvement agents, and the like. The water-absorbent resin is used particularly for hygiene products among the aforementioned use purposes.
Such water-absorbent resin is typically lightly-crosslinked copolymer. As water-absorbent resin, starch-based water-absorbent resin such as a starch-acrylonitrile graft copolymer hydrolysate (see Patent Literature 1) and a starch-acrylic acid graft copolymer neutralized product (see Patent Literature 2), a vinyl acetate-acrylic acid ester copolymer saponified product (see Patent Literature 3), a partially neutralized products of polyacrylic acid (see Patent Literatures 4, 5, and 6), and the like are known, for example.
As properties that the water-absorbent resin is to exhibit, an excellent water absorption property, an excellent handling property, and the like are exemplified. The excellent water absorption property means that an absorption capacity, an absorption rate, and liquid permeability when water-based liquid such as body fluid is brought into contact with the water-absorbent resin are satisfactory. In addition, the excellent handling property means that the water-absorbent resin exhibits excellent flowability even if the water-absorbent resin has absorbed moisture and only a small amount of water-absorbent resin adheres to equipment for manufacturing an absorbent material when the absorbent material is produced by using the water-absorbent resin and a fibrous material. Furthermore, the water-absorbent resin has been required to have additional functional properties such as an antibacterial property and a deodorizing property as a demand for disposable diapers for adults increases with the aging of the population.
Techniques of adding additive particles to water-absorbent resin particles have been proposed for the purpose of improving such a water-absorption property, a handling property, and additional functional properties. For example, techniques for improving anti-gel blocking property and liquid permeability when a diaper absorbs urine (see Patent Literatures 7 to 9), techniques for improving anti-blocking property under moisture absorption (see Patent Literatures 10 to 15), and techniques for providing an antibacterial property and a deodorizing property (see Patent Literatures 16 to 19) have been proposed. In addition, a technique of adding water-soluble metal soap powder to water-absorbent resin particles (see Patent Literature 20) and techniques of adding multivalent metal salt powder to water-absorbent resin particles (see Patent Literatures 21 and 22) have been proposed.